This article sums up my thoughts fairly well so I’ll let it do my talking.
The series started out well but since then both the books and TV series have gone downhill for me. Season 5 kind of put the icing on the cake. The series and books sprawl from here to there showing unimportant or irrelevant story-lines (e.g. Arya up to season 5), abandoning characters or ending plot-lines like blind alleys, etc.
It’s like they’ve forgotten the point of the exercise; to entertain or at least tell a coherent story. The worst offences were evident this season where I noted that the series has actually become predictable in finding ways to disappoint the viewer.
A story is not as entertaining when it’s just an endless bag of sadness.
You can only keep that up for so long before the viewer becomes uninterested and wanders off.